Classification of Organic Compounds

There are over 27 million chemical compounds known. As a result, studying them separately is nearly impossible. Thus there is a need for the classification of organic compounds.

Organic substances can be categorized in two ways structurally and chemically i.e.,

• Based on structure
• Based on functional group

A general classification of the organic compound on the basis of structure is described in the below image

Acyclic  compounds

These are the compounds formed by joining carbon atoms to form an open chain. Aliphatic compounds are another name for these chemicals. 

Their structure might be either straight or branched chains.

Example of straight-chain,

• n-Butane: CH₃​−CH₂​−CH₂​−CH₃

Example of branched-chain,

• Isobutane:

Isobutane

Cyclic compounds

These are the compounds in which carbon atoms are joined to form one or more rings with or without heteroatom. 

They are further classified into two types as homocyclic and heterocyclic compounds.

• Homocyclic or carbocyclic: In these compounds, the ring is made up of carbon atoms only. Homocyclic compounds are further divided into two types:

1. Alicyclic compounds: These compounds show some of the properties similar to those of aliphatic compounds. In these homocyclic compounds, carbon atoms are linked by single bonds only. e.g. Cyclopropane, etc. 
2. Aromatic Compounds: These compounds contain at least one aromatic ring which resembles benzene in their chemical behavior. e.g. Benzene, etc.
   • Aromatic compounds are further divided into benzenoid and non-benzenoid compounds. At least one benzene ring is present in the structure of benzenoid compounds and Non-benzenoid substances have an aromatic ring that is not benzene. Thus.

• Heterocyclic compounds: These compounds include one or more heteroatoms like oxygen, nitrogen, sulfur, etc. in the ring of carbon atoms. They are also further divided into two types.

1. Hetero-alicyclic compounds: Alicyclic compounds which contain at least one heteroatom in the ring are called hetero-alicyclic compounds. e.g. Tetrahydrofuran (THF), etc.
2. Hetero-aromatic compounds: Aromatic compounds which contain at least one heteroatom in the ring are called hetero-aromatic compounds. e.g. Furan, etc.
   • Hetero-aromatic compounds are classified as benzenoid compounds and non-benzenoid compounds as,

Classification of Organic Compounds on the basis of functional group

The functional group is the part of a molecule that influences its chemical characteristics. Every organic molecule’s chemistry, regardless of size or complexity, is defined by the functional group it contains.

Organic compounds are categorized as follows based on their functional group 

S. no.	Class	General Formula	Functional Group	Common Suffix/prefix (systematic)
1	Hydrocarbons
	Alkanes	R – H	C – C	-ane
	Alkenes	R – CH = CH – R	C = C	-ene
	Alkynes	R – C ≡ C – R	C ≡ C	-yne
	Arenes	ArH		-ene
2	Halogen containing compounds
	Alkyl Halides	R – X	– X	-halide
	Aryl Halides	ArX		halo-
3	Oxygen-containing compound
	alcohols	R – OH	– OH (hydroxyl)	-ol
	Phenols	Ar – OH	– OH (Phenolic)	-ol
	Ethers	R – O – R’	– C – O – C –	ether
	Aldehydes	RCHO		-al
	Ketones	RR’C = O		-one
	Carboxylic acid	RCO2H		-ic acid
4	Carboxylic acid derivatives
	Esters	RCO2R’		-ate
	Amides	RCONHR’		-amides
5	Nitrogen-containing compound
	Amines	RNH2 , RNHR’	-NH2 (Amino)	-amine
	Nitriles	RC ≡ N	– C ≡ N	-nitrile
	Nitro Compounds	ArNO2		nitro-
where, R – Represents Alkyl group and  Ar – represents Aryl group

Homologous Series

A homologous series is a set of organic compounds that each include a distinct functional group and differ from one another in the molecular formula by the a-CH₂-(methylene) group.

Individual series members are referred to as homologs, and they can be represented by a generic molecular formula. Homologs are ordered in increasing order of molecular mass in homologous series.

The General molecular formula is, C_nH_{2n + 1} (Functional group)

For Example, the homologous series of alcohol is

Member	Name
CH3OH	Methyl alcohol
C2H5OH	Ethyl alcohol
C3H7OH	Propyl alcohol
C4H9OH	Butyl alcohol

NOTE: Ethyl alcohol is higher homologous of methyl alcohol but a lower homolog of propyl alcohol

Characteristics of homologous series

The following are the characteristics of homologous series:

• The same general formula may be used to express homologs.
• Homologs are members of the same functional group.
• Because the consecutive components’ chemical formulas change by -CH2-group, their molecular weights differ by 14 units.
• Homologs can be synthesized using comparable chemical processes.
• Homologs have chemical characteristics that are identical.
• Physical characteristics such as melting point, boiling temperature, density, solubility, and so on vary across homologs.

Sample Questions

Question 1: Write the name of the class of the following organic compound

• CH₃ – (CH₂)₃ – CH₂ – Cl
• CH₃ – CH₂ – CH₂ – NH₂
• CH₃ – CH₂ – OCH₃

Answer: 

• The first compound contains an halogen atom (-Cl) thus, its an alkylhalide
• The second compound contains amino group(-NH₂) thus its an amine
• The third compound contains a ether group thus its an ether.

Question 2: Write the complete structural formula of the following organic compound

• 4-Ethyl-3 methyl octane
• 2-Methylbutan-2-ol
• 2-Methylcyclohexa-1, 3-diene

Answer:

4-Ethyl-3 methyl octane

2-Methylbutan-2-ol

2-Methylcyclohexa-1, 3-diene

Question 3: Write a note on the homologous series and write the general molecular formula?

Answer:

Homologous series is a series of organic compounds containing characteristics functional group and the successive member differ from each other by a -CH₂ group.

Organic compounds form homologous series such as alkanes, akenes, alknes, alcohol, amines, etc.

General Molecular Formula: C_nH_{2n + 1} (Functional group)

Question 4: Write the name and molecular formula of the first six homologous of homologous series of alkyl chlorides.

Answer:

Molecular formula C_nH_{2n + 1} X

The first 6 homologous series of alkyl chloride are

1. CH₃Cl  (methyl chloride)
2. C₂H₅Cl  (ethyl chloride)
3. C₃H₇Cl  (propyl chloride)
4. C₄H₉Cl  (butyl chloride)
5. C₅H₁₁Cl  (pentyl chloride)
6. C₆H₁₃Cl  (hexyl chloride)

Question 5: Write the structural formula of the first 4 members of the homologous series of the following compounds

• Aldehydes
• Carboxylic acid

Answer: 

Aldehydes

1. H -CHO (methanal)
2. CH₃ – CHO (ethanal)
3. CH₃ – CH₂ – CHO (propanal)
4. CH₃ – CH₂ – CH₂ – CHO (butunal)

Carboxylic acid

1. H – COOH (methanoic acid)
2. CH₃ – COOH (ethanoic acid)
3. CH₃ – CH₂ – COOH (propanoic acid)
4. CH₃ – CH₂ – CH₂  – COOH (butanoic acid)

Question 6: Name the following organic compounds. List the names of functional groups in each compound

Answer:

1. 4-Hydroxypent 2-enal [Functional group – hydroxy, aldehyde, alkene]
2. 4-amino- 4-methylpentan-2 one [Functional group – Amino, keto]
3. 2-3 dimethyl butane-2-3 diol [Functional group – hydroxy]